(1) Field of the Invention
This invention relates to the fabrication of integrated circuit devices and more particularly to a method of local oxidation using a salicide process in the fabrication of integrated circuits.
(2) Description of the Prior Art
Local oxidation of silicon is the conventional lateral isolation scheme. The conventional local oxidation process is described in VLSI Technology, International Edition, by S. M. Sze, McGraw-Hill Book Company, NY, N.Y., c. 1988 by McGraw-Hill Book Co., pp. 473-474. A layer of silicon nitride is deposited over a pad oxide (a thin thermal oxide which allows better adhesion between the nitride and silicon) overlying a silicon substrate. The nitride and oxide layers are etched to leave openings exposing portions of the silicon substrate where the local oxidation will take place. A boron channel-stop layer is ion implanted into the isolation regions. The field oxide is grown within the openings and the nitride and oxide layers are removed. This completes the local oxidation.
On pp. 476-477 of the aforementioned textbook, Sze describes some of the disadvantages of the growth of field oxide using the local oxidation method. He says that field oxidation can cause lateral diffusion of the channel-stop layer which raises the surface concentration of the substrate near the edges of the field oxide. This causes an increase in the threshold voltage of those portions of the active devices. The edges will not conduct as much as will the interior; therefore, the transistor will behave as if it were narrower than it is.
Another problem with the local oxidation process is that the field oxide will penetrate under the masking nitride layer causing the space between transistors to grow during oxidation. This oxide growth under nitride is called "bird's beak encroachment."
Other problems include stress in the oxide in the region covered by the nitride mask and a non-flat surface after oxidation.
These problems are addressed in the present invention by incorporating a salicide recessing process into the local oxidation process. The salicide method is well-known. It is used to build up an insulating layer between source and drain regions and gate electrodes to avoid short circuits. A metal is deposited over the integrated circuit and heat-treated to cause the metal to react with the polysilicon of the gate and the monosilicon of the source/drain regions. The metal does not react with underlying silicon oxide or silicon nitride layers. The metal that has not reacted is etched away leaving the silicide insulating areas. U.S. Pat. Nos. 4,855,247 to Ma et al and 4,912,061 to Nasr are examples of patents describing salicide processes. The present invention applies a salicide method in a novel approach to local oxidation.